The principles of active sound and vibration control have been known for many years and there is a wealth of published literature. Most patent specifications in this field relate to methods applicable to particular situations. The method and system described herein relate to the control of periodic or almost periodic sound and vibration. The two main approaches to this problem are:
(i) Wave shaping or filtering, eg U.S. Pat. No. 4,506,380 and published UK Patent Application No. 2,201,858, where a reference signal containing one or more frequencies of the unwanted sound and vibration is filtered to produce the signals to send to actuators which in turn produce the desired sound or vibration.
(ii) Waveform synthesis, where a waveform-generator istriggered by a signal derived from the source, eg UK Patent Specification No. 1,577,322.
The two methods are equivalent only if the vibration source is exactly periodic. If the source characteristics are changing in time it is usual to use an adaptive control system in which sensors in the region to be controlled sense the residual sound or vibration and pass the information to a processor which alters the filter coefficients or the synthesized waveform so as to provide better control. Published UK Patent Application No. 2,201,858 describes methods for adapting filter coefficients. UK Patent Specification No. 1,577,322 recognises the need for adaption and a later patent specification, UK Patent No. 2,107,960, describes a simple technique for such a system using a single actuator and sensor. This latter patent specification does not explain how to control vibration where the period is changing, except to suggest that in this case the transform technique should produce frequency components from the lowest expected frequency to the highest, rather than just at frequencies corresponding to the harmonics of the source.
A further Patent Specification No. 2,122,052, uses a waveform synthesis technique for vibration control. In this method a sensor and actuator are placed at each of a number of locations. This results in a system with equal numbers of sensors and actuators and a method for adapting the waveform is presented for this special case. In most applications, however, the sources and sensors are not colocated and usually more sensors than sources are used in an effort to obtain a better measure of the resulting sound or vibration.